The present invention is directed to a bore mapping and surface time measurement system for a bore hole in a material such as metal and, more particularly, the present invention is directed to a bore mapping system which maps the diameter of a power generating plant turbine rotor bore and measures the time required for an ultrasonic wave to reach the bore surface for later use to correct the transit time of an angled beam refracted by the bore surface to create a shear mode interrogation beam during a shear mode ultrasonic bore inspection.
Power generating station turbines and generator rotors have variable diameter boreholes extending along their central axis over part or all of their lengths. Because the rotors experience the highest stress within approximately the first four inches in rotor radius, the detection of major near bore flaws and the monitoring of flaw location as well as size determinations are of critical importance in determining rotor life expectancy. As a result, at the end of the manufacturing process and during periodic routine rotor maintenance, the bores are inspected for flaws and occasionally remachined over all or part of their surface to remove the flaws. To accurately perform the inspection, the precise geometry in the bore of a particular rotor must be determined. Prior to a rotor inspection, the inside diameter of the bore, as a function of axial position along the bore, needs to be determined to allow inspection transducers to be properly calibrated and manipulated during the inspection.
One method of obtaining bore diameter information is to manually insert a mechanical measurement device into the bore and record the diameter at discrete locations along the length of the bore. One such mechanical measurement device is called a "Star Gauge". The "Star Gauge" measures diameter by mechanically positioning arms inside the bore. The mechanical system suffers from drawbacks related to limited axial resolution and inevitable human error associated with gauge positioning. In addition, such mechanical measuring devices are difficult to align on bore slopes and tapers.
The ultrasonic inspection of the rotor bore involves transporting an ultrasonic transducer through the bore and consists of a combination of circumferential rotation and axial displacement allowing the ultrasonic beam to pass through all of the material of interest. In an immersion type system, the inspection transducer does not contact the bore surface but operates at a distance from the bore. An immersion fluid, such as treated water, provides the necessary sound transmission medium between the transducer and the rotor material. During each rotation and/or axial movement of the transducer and its scan head, the path length travelled by the ultrasonic wave between the transducer and the bore surface can vary due to several factors: (1) the wobble, run-out, or eccentricity of the scan head relative to the bore surface; (2) twisting of the scan head used to transport the transducer; and (3) misalignment of the entire scan head upon entry into the bore.
The location of detected reflectors (flaws) in the rotor material are determined by calculation based on a measured ultrasonic wave propagation or travel time from the transducer to the flaw and back, transducer location at the time of flaw detection and a knowledge of the geometry of the ultrasonic beam path. The propagation velocities of the ultrasonic wave or pulse in the liquid coupling medium and the rotor material are different and the actual path of any wave is composed of a water path component and a metal path component. Only the total time is available as a measurement, thus, the water path component must be precisely known in order to determine the metal path component. It is from the metal path component that the depth of the flaw from the bore surface is calculated. Reference or calibration blocks or bores containing known size reflectors of known geometry at known locations, can be scanned prior to rotor inspection to calibrate the transducers by a procedure in which the geometry of the ultrasonic path is determined to thereby establish the relationship between the inspection transducers and later discovered flaws. If the path length and corresponding surface time of the ultrasonic pulse during an inspection have changed from the path length and surface time measured during calibration, and this change has not been accounted for, the location of the flaws in the rotor material will be incorrectly determined. In extreme cases, reflections originating from within the material can be interpreted as air bubbles trapped in the immersion fluid, or bubbles in the immersion fluid can be interpreted as surface or subsurface reflections from material discontinuities or flaws in the rotor itself. This problem is of particular significance when the ultrasonic beam is not normally incident to the bore surface, as is the case for shear mode inspections.
During a normally incident compressional mode inspection, the ultrasonic beam is directed at the bore surface in a perpendicular or nearly perpendicular direction and produces a strong surface reflection back to the transducer, and the signal from this return echo can be used to locate the surface of the bore from which flaw positions can be measured. In shear mode testing, however, the shear wave is produced by directing a compressional wave in the immersion fluid toward the bore surface at an angle relative to the radial direction, that is, the compressional wave strikes the bore surface at an angle other than perpendicular. This angle is normally about 21.degree. so that a refracted shear wave is created in the rotor material at an angle of about 45.degree. to the surface tangent at the pulse contact point. At this angle of incidence, the surface echo is reflected away from the transducer incident beam path. Thus, no signal is returned to the transducer, and the water path component of the inspection beam cannot be determined from transducers used for shear wave inspection.
Prior art immersion based ultrasonic inspection systems employ a type of bore riding immersion transducer which is mounted in a housing which itself is kept in contact with the bore thereby maintaining the transducers at fixed offsets with respect to the bore surface. The drawbacks of this prior art system are based on two contradictory requirements for a bore riding transducer. The transducer housing must be compliant radially so that the housing properly tracks variations in the bore and yet the support mechanism must be rigid enough so that the compliance does not lead to positional errors.
If the surface time at which the shear mode ultrasonic beam contacts the bore surface is known as a function of circumferential position, the position reporting errors which can occur in a shear transducer inspection system mentioned above, can be corrected if required. Accurate surface time information can also be used to eliminate confusion regarding the source of near bore reflectors or flaws located just inside the bore surface. The first order correction assumes that the geometry of the ultrasonic beam path inside the rotor material has not changed since calibration, and that the change in surface time is due wholly to a change in coupling fluid path length. In practice, the first order correction is the most important for properly locating flaws near the rotor bore surface. Higher order corrections of the refracted beam path can also be performed if the correct surface time is known as a function of circumferential position.